Electrophotographic or laser printers are presently common in the office workplace. In basic terms, a laser printer typically includes a formatter, a laser driver, a laser diode and a photosensitive drum. The laser printer generally receives print commands and character and graphic data in digital form from an external computer via a computer interface cable. The formatter processes the character and graphic data to generate a video signal for controlling the laser diode. The video signal is transmitted from the formatter to the laser driver via ribbon cables. Based upon the video signal, the laser driver generates a drive current that energizes the laser diode. In response, the laser diode generates a laser beam that strikes the photosensitive drum to create a charge pattern corresponding to the image to be printed. The charge pattern is then developed and transferred to a print medium, such as paper.
The video signal is a rasterized representation of the image to be printed. The formatter processes the externally received character and graphic data with a process sometimes referred to as bit mapping to break the image down into a matrix of pixels. The matrix of pixels is divided into scan lines, and the pixels are represented serially in each scan line by the video signal. The image is generated using the video signal to process the pixels line-by-line. In a single-tone laser printer, each pixel location is either on or off, i.e., either dark or light. The video signal typically comprises a train of pulses indicating whether each of the pixel locations is dark or light. More specifically, each pixel location is represented by a particular portion of the video signal. For example, 100 nanoseconds of the video signal could be dedicated to each pixel location. Whether the pixel should be dark or light is represented by the voltage level of the video signal. For example, each dark pixel could be represented by a high voltage level and each light pixel could be represented by a low voltage level. Then, if a particular pixel location should be dark, the portion of the video signal corresponding to that pixel would contain a high voltage level pulse that would activate the laser diode.
Recently, gray-scale laser printers have been introduced, making it possible to represent shades of color in an image produced by the laser printer. One way of representing the shade of gray at a particular pixel location is by varying the width of the pulse within the portion of the video signal dedicated to the pixel location. For example, no pulse could represent white, i.e., no color, and pulses of increasing width could represent darker shades of gray. In essence, in this manner, a gray-scale video signal is achieved by the use of pulse-width modulation. Because different pulse widths represent different shades of gray, it is important that the pulse width received by the laser driver be nearly the same as the pulse width originally generated by the formatter.
Various factors can affect the intended pulse width in the signal received by the laser driver, including the type of conductor used to convey the signal from the formatter. Transmitting the video signal from the formatter over ribbon cables to the laser driver can distort the widths of the pulses so that the image is corrupted, i.e., pixel locations are darker or lighter than intended. The sensitivity to pulse width distortion increases as the number of shades in the gray scale is increased because the difference between the pulse width representing one shade and the pulse width representing the next shade decreases. Sensitivity is further increased as the video signal is shortened in time to increase the speed of the laser printer. That is, as each pixel is represented by a smaller portion of the video signal, the sensitivity to variations in pulse width increases.
Unfortunately, interfaces currently used to interconnect a formatter and laser driver do indeed distort the video signal, including distortion of the pulse widths. A ribbon cable typically interconnects the output of the formatter to the input of the controller, and the output of the controller is typically connected to the input of the laser driver with a second ribbon cable. The controller may include test functions for testing the integrity of a video signal during test operations. Buffers, e.g., digital gates such as an AND gate, are often included in the formatter output section and in the controller, to drive the video signal across the ribbon cables. Without these buffers, the video signal quality would deteriorate over the length of the ribbon cables to an unacceptable level. For example, the edges of pulses would begin to "flatten" out as the signal propagated over the ribbon cables. The signal source output impedance takes time to charge the capacitive load of the ribbon cable transmission line. In essence, a buffer restores or reconditions the signal received at its input. For example, if the pulse arriving at the input of a buffer has sloping edges, the output amplification of the buffer generates a nearly square pulse with vertical edges. Thus, to avoid deterioration of pulses, the signal path from the formatter to the laser driver includes a series of buffers.
Unfortunately, the use of series connected buffers creates another problem. Generally, the rise and fall times of a particular buffer are not the same (especially over voltage, temperature, and buffer process variations), causing the width of a pulse to become distorted, i.e., shortened or widened, upon passing through the buffer. As the video signal passes through a series of buffers, this problem is magnified. The pulse width distortion can increase to the point that the pulse arriving at the laser driver represents a darkness several shades different from that originally transmitted. As a result, the resulting image is inaccurate. In addition to the buffers distorting the video signal, electromagnetic noise within the laser printer can create cross-talk on the video-signal interface that further corrupts the video signal.
The present invention provides a new video-signal interface that addresses these and other problems, by substantially cancelling the distortion caused by buffers and electromagnetic noise.